High carbon steel wire with bainitic structure for spring and other cold-formed applications

ABSTRACT

According to the present invention, there is disclosed a heat treated high carbon or alloy steel wire and method of heat-treating the wire. The wire is produced by the steps of initially heating the high carbon or alloy steel wire to a temperature of about 1600 degrees F. to about 1800 degrees F. for between 10 seconds and 2 minutes; rapidly cooling the wire to an intermediate point temperature of between 400 to about 1000 degrees F.; holding the wire at the intermediate point temperature for about 2 minutes to about 40 minutes; and cooling the resulting wire to ambient temperature.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application No. 60/385,079, filed May 31, 2002 by Bhagwat et al.

TECHNICAL FIELD

The present invention relates to the manufacture of wire springs and other wire forms and more particularly to the method of heat treatment of continuous lengths of steel wires for the manufacturing of springs and wire forms that results in a bainitic microstructure in the steel wire.

BACKGROUND OF THE INVENTION

Conventionally, spring wire is formed from rod or wire that is drawn to size and in some cases, further rolled to different shapes. Next, the sized wire is heat treated, for example to an austenitic temperature of about 1650° Fahrenheit (F). Then, the heat-treated wire quenched, i.e., in an oil bath at about 200° F., to a temperature of about 300° F. Continuing with the conventional process, the heat quenched wire is then tempered by reheating to a temperature, for example of about 850° to 900° F., and held for sufficient time to increase the ductility, and finally allowed to cool to ambient temperature. The result is tempered martensite structure in steel wire that is used to form springs and wire forms.

The problem is that the springs and wire forms constructed of tempered steel wire made in accordance with the conventional process described herein before often requires additional processing steps during manufacture or service. Because of the processing steps, these steel wires have low ductility and are prone to inferior coiling performance. The microstructure of martensitic wires also limits the tensile strength and thus, the load bearing capacity of the springs and wire forms so made. In the case of flat wire coiled springs, when these springs are produced, the end of the steel wire is typically reheated, especially at the wire ends to bend the end to form a center bar. The reheating coarsens the martensitic structure that leads to a weakness at the bends. Also, the reheating is an additional step that increases the production cost of the coil spring. Further, the steel chemistry and the temperature of the tempering process is controlled so that the steel wire has a hardness of about 42-46 Rockwell C(RC). Hardness above RC 46, without changing the steel chemistry, would result in a wire that is too brittle to form a coil.

It would be desirable, however, to have a steel wire with high ductility and a higher hardness, Rockwell C, to form a spring or wire form for enhanced formability and fatigue life of the spring and wire, respectively. It would also reduce the required wire size and thereby, the spring weight.

OBJECTS OF THE INVENTION

It is an aspect of the present invention to a provide spring wire, preferably in continuous lengths, for forming a spring coil or wire form which provides one or more of the following:

-   -   a. An innovative heat treatment process to produce a wire with         bainitic structure that provides increased tensile strength         while retaining the high level of ductility;     -   b. Significant improvement in spring fatigue strength or wire         form fatigue strength;     -   c. Elimination of the need for expensive alloy steels for         moderately elevated temperature strength retention—viz.         hot-hardness;     -   d. Increased productivity because of the elimination of the need         for stress relieving treatment by reheating the coil or wire         form to an elevated temperature;     -   e. Elimination of the need of annealing the wire before forming         and consequently lowering the spring or wire form production         cost; and     -   f. Elimination of subsequent austempering treatment of springs         after the hard-rolled or hard-drawn wire is coiled.

SUMMARY OF THE INVENTION

According to the present invention, there is disclosed a method of heat-treating a high carbon or alloy steel wire comprising the steps of: initially heating the high carbon or alloy steel wire to a temperature of about 1600 degrees F. to about 1800 degrees F. and preferably between 1625 degrees F. and 1700 degrees F. for between 10 seconds and 2 minutes; rapidly cooling the wire to an intermediate point temperature of between 400 to about 1000 degrees F. and preferably between 550 and 950 degrees F.; holding the wire at the intermediate point temperature for about 2 minutes to about 40 minutes; and cooling the resulting wire to ambient temperature.

Also according to the present invention, the heat treated wire is held at the intermediate temperature is for between 10 and 20 minutes for complete lower bainitic transformation.

Further according to the present invention, the heat treated wire has a fully lower bainitic microstructure and a tensile strength of between 160 and 290 ksi and preferably a tensile strength of between 200 and 270 ksi.

Still further according to the present invention, the heat treated wire has a fully lower bainitic microstructure and a yield strength of between 130 and 242 ksi.

Also according to the present invention, the heat treated wire has a hardness of between 32 and 56 Rockwell C.

Further according to the present invention, the heat treated wire has a high ductility as measured by reduction in area of about 35% to about 60%.

Also according to the present invention, the heat treated wire is adapted for forming a coil spring and wire forms.

According to the present invention, there is also disclosed a high carbon steel, heat treated wire comprising a high carbon steel wire having a lower bainitic microstructure and a tensile strength of between 160 and 290 ksi, a yield strength of between 130 and 142 ksi and a high ductility as measured by reduction in area of about 35% to about 60%.

Further according to the present invention, the heat treated wire has a tensile strength of between 200 and 270 ksi, a yield strength of between 175 and 230 ksi and a high ductility as measured by reduction in area of about 35% to about 60%.

According to the present invention, there is also further disclosed a wire produced by the process of heat-treating a wire high carbon or alloy steel comprising the steps of: initially heating the high carbon or alloy steel wire to a temperature of about 1600 degrees F. to about 1800 degrees F.; rapidly cooling the wire to an intermediate point temperature of between 400 to about 1000 degrees F.; holding the wire at the intermediate point temperature for about 2 minutes to about 40 minutes; and cooling the resulting wire to ambient temperature.

Also according to the present invention, the heat treated wire includes the step of initially heating the wire to a temperature of between 1625 degrees F. and 1700 degrees F.

Further according to the present invention, the heat treated wire includes the step of in-process cooling wherein the wire is cooled to an intermediate point temperature of between 550 and 950 degrees F.

Also according to the present invention, the step of initially heating high carbon or alloy steel is for between 10 seconds and 2 minutes and the wire is held at the intermediate temperature is for between 10 and 20 minutes for complete lower bainitic transformation.

BRIEF DESCRIPTION OF THE DRAWINGS

The structure, operation, and advantages of the invention will become further apparent upon consideration of the following description taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a cross-sectional view of a flat wire coil spring; and

FIG. 2 is a cross-sectional view of a compression spring.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention relates to a process that results in an alternative microstructures as compared to the microstructure that results from the conventional process of quenching and tempering steel wires that are typically used to form coil springs, torque rods, and other wire forms of the type shown in FIGS. 1 and 2. However, it is also within the scope of the present invention to use the steel wire formed in accordance with the principles of the present invention to form other objects of manufacture besides the wire forms and coil springs as discussed herein.

The present invention utilizes a series of processing steps that results in the formation of steel wire with improved mechanical properties as compared with the mechanical properties of the steel wire made with the prior art conventional process used to form coil spring steel wire with a hardness of about 42-50 Rockwell C, as discussed herein before. The improved steel wire made in accordance with the process of the present invention also has improved performance characteristics, especially when applied to manufacturing of a coil spring. The difference in performance arises from the metallurgical difference in the microstructure of the heat-treated wires. The improved spring wire of the present invention has a lower bainite microstructure instead of the tempered martensite microstructure of the spring wire formed by the tempering process of the prior art.

The steel wire with a lower bainitic microstructure has a higher ductility as measured by Reduction in Area of between 35% and 50% and preferably between 40% and 45%. Further, the steel wire made in accordance with the process of the present invention has a relatively high hardness of between 32 and 56 Rockwell C and preferably between 46 and 52 Rockwell C. Consequently, the steel wire made in accordance with the process of the present invention has a higher tensile strength and yield strength. The steel wires made in accordance with the process of the present invention can be formed with round, flat or other shaped cross sections and are adapted for use in a wide range of spring and other wire form applications. The invention also allows the spring wire formed in accordance with the principles of the present invention to achieve higher performance while reducing the number of processing steps.

According to the processing steps of the present invention, a relatively common grade of high carbon steel wire, such as AISI 1080, is initially heated to a temperature of between 1600 degrees F. and 1800 degrees F. and preferably between 1625 degrees F. and 1700 degrees F., and most preferably about 1650 degrees F. The wire is held at this temperature for between 10 seconds and 2 minutes. Then, the wire is cooled rapidly to an intermediate temperature of between 550 and 950 degrees F. and preferably between 600 and 700 degrees F., and most preferably about 650 degrees F. After the wire is held at this intermediate temperature for between 2 and 40 minutes, and preferably between about 10 to about 20 minutes and most preferably about 15 minutes for complete lower bainitic transformation, the wire is then allowed to cool to ambient temperature in air. The tensile strength of the resulting wire is between 160 and 290 kilopounds per square inch (ksi), and preferably between 200 and 270 ksi, and most preferably about 250 ksi. Further, the wire has yield strength between 130 ksi and 242 ksi, and preferably between 175 and 230 ksi, and most preferably about 205 ksi. Further, the wire has a high ductility of between 35% and 60% reduction in area and preferably about 40% reduction in area. This latter ductility value is similar to that of pearlitic structure.

In the case of regulator springs, one of the advantages of forming the regulator spring from a continuous length of lower bainite structure wire is that it does not require that the ends be annealed during forming as with the prior art. This means that processing steps and cost of production are reduced. Further, no post-forming, stress relief of the springs is required because of lower residual forming stresses in bainitic structures than in previously used tempered-martensitic wire. Also, the higher ductility of the structure allows use of steel with higher impurities, and therefore, may be less costly. Moreover, the wire can be coiled at a high speed of between 30 and 500 feet per minute without breakage.

Generally, as shown in FIG. 1, in making a spring coil 12 from steel wire, the end 14 of the wire 10 is inserted into a conventional spring coiling machine (not shown) and attached to a center shaft (arbor) by bending the end 14 to form a straight section 16 called the center bar. Then, the wire 10 is pulled into the conventional spring coiling machine (not shown) at a constant speed. As the wire 10 is fed into the spring coiling machine at a constant speed, the center ring 18 a of the coil spring 12 is wound into a coil shape more quickly than the outer rings are wound. This is because as the coil becomes larger (bigger outer diameter), more wire is needed to form each successive outer ring. In other words, as the diameter of the coil spring increases, each successive outer ring is being wound into a coil shape slower than the adjacent, previous inner ring, i.e. 18 d is wound into a coil faster than 18 c. The result is that the strain rate of each ring 18 b . . . 18 f is lower than the adjacent inner ring as the coil spring 12 is wound. The decrease in strain rate from the center coil ring 18 a to the outer coil ring 18 f leads to a transition zone of low and high strain rates with different dynamic strain aging temperatures. Thus, the coil rings 18 b . . . 18 f have incompatible elongation properties which are not equalized across the coils 18 a-18 f at the stress-relieving temperature, i.e. the annealing temperature to which the coil spring 12 is subjected to after being formed. By using the apparatus and methods of the prior art, the wire coil would often break in ring 18 b opposite from the straight section 16, as shown in FIG. 1.

According to the present invention, the spring wire has a lower bainite microstructure which is ductile and doesn't require preheating of the spring ends to form the hook. Thus, the possibility of dynamic strain-aging is alleviated. Further, the lower bainite wire, being more ductile, doesn't require post-forming stress relieving of the wire coil

Some of the advantages of the present invention are:

-   -   a. An innovative heat treatment process to produce a wire with         lower bainitic structure and increased tensile strength, yield         strength, and ductility;     -   b. Significant improvement in fatigue strength of a spring or         wire form;     -   c. Reduced softening of the wire at moderate temperatures;     -   d. Reduced production cost because of the elimination of the         need for annealing and post-forming stress-relieving; and     -   e. Lower material and processing cost.

EXAMPLE

A steel wire (AISI 1082 grade) of size 0.125″×0.500″ was heat-treated to produce continuous lengths of wire with lower bainitic structure. The chemical analysis of the AISI 1082 grade steel in weight % was as follows: Carbon 0.82% Manganese 0.51% Silicon 0.23% Processing Background:

-   -   1. Steel rod was drawn to a required round size and then rolled         to 0.125″ thick and 0.500″ wide rectangular wire.     -   2. The wire was then austempered to achieve a bainitic structure         with a hardness of Rockwell C scale 50. The resulting product         was continuous length of wire in coil form that would enable to         produce springs in plurality.     -   3. The processing parameters for the austempering treatment were         selected such that the resulting structure was lower bainite.         Particularly, the wire was first heated to 1700° Fahrenheit         (925° Centigrade), followed by rapid cooling to avoid         transformation to pearlite and cementite. In the same sequence         of heat treating process, the wire was then held at 570°         Fahrenheit (300° Centigrade) to transform the unstable         austentite to lower bainite.     -   4. The resulting structure yielded a hardness of Rockwell C 50,         tensile strength of 255 ksi, and yield strength of 208 ksi.         The wire was further coiled to verify spring coiling performance         as described below.     -   1. The wire was coiled to form a belt tensioner spring by a         spring manufacturer.     -   2. The spring manufacturing process was identical to the process         of the prior art. Therefore, no difference was evident to the         spring manufacturer in spite of the higher strength of the         material. Hence, the present invention did not necessitate any         changes in the spring manufacturing process over the prior art.         Service Performance of Springs:     -   1. Eight springs manufactured according to the present invention         were compared with four springs manufactured according to the         prior art. All of the springs were produced at the same time and         with identical manufacturing settings. Springs from the present         invention showed identical dimensional characteristics while         those made by the wire formed according to the prior art process         did not have identical dimensional characteristics.

While the invention has been described in combination with embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing teachings. Accordingly, the invention is intended to embrace all such alternatives, modifications and variations. 

1. A method of making a bainite wire coil spring comprising the steps of: a. initially heating a continuous length of high carbon or alloy steel wire to a temperature of about 1600 degrees F. to about 1800 degrees F.; b. rapidly cooling the continuous length of wire to an intermediate point temperature of between 400 to about 1000 degrees F.; c. holding the continuous length of wire at the intermediate point temperature for about 2 minutes to about 40 minutes; d. cooling a resulting continuous length of lower bainite heat treated wire to ambient temperature; and e. winding the continuous length of lower bainite heat treated wire into a coil spring.
 2. The method of making a bainite wire coil spring of claim 1 wherein the step of initially heating the continuous length of wire is to a temperature of between 1625 degrees F. and 1700 degrees F.
 3. The method of making a bainite wire coil spring of claim 1 wherein the step of in-process cooling the wire includes cooling the continuous length of wire to an intermediate point temperature of between 550 and 950 degrees F.
 4. The method of making a bainite wire coil spring of claim 4 wherein the step of in-process cooling the wire includes cooling the continuous length of wire to an intermediate point temperature of about 650 degrees F.
 5. The method of making a bainite wire coil spring of claim 1 wherein the step of initially heating the continuous length of high carbon or alloy steel wire is for between 10 seconds and 2 minutes.
 6. The method of making a bainite wire coil spring of claim 1 wherein the step of wire holding the continuous length of wire at the intermediate temperature is for between 10 and 20 minutes for complete lower bainitic transformation.
 7. The method of making a bainite wire coil spring of claim 1 wherein the resulting lower bainite wire has a fully lower bainitic microstructure and a tensile strength of between 160 and 290 ksi.
 8. The method of making a bainite wire coil spring of claim 7 wherein the resulting lower bainite wire has a fully lower bainitic microstructure and a tensile strength of between 200 and 270 ksi.
 9. The method of making a bainite wire coil spring of claim 1 wherein the resulting lower bainite wire has a fully lower bainitic microstructure and a yield strength of between 130 and 242 ksi.
 10. The method of making a bainite wire coil spring of claim 1 wherein the resulting lower bainite wire has a hardness of between 32 and 56 Rockwell C.
 11. The method of making a bainite wire coil spring of claim 1 wherein the resulting lower bainite wire has a high ductility as measured by reduction in area of about 35% to about 60%.
 12. (canceled)
 13. A high carbon steel, heat treated bainite wire coil spring comprising; a continuous length of high carbon steel, heat treated wire having a lower bainitic microstructure and a tensile strength of between 160 and 290 ksi, a yield strength of between 130 and 242 ksi and a high ductility as measured by reduction in area of about 35% to about 60%.
 14. (canceled)
 15. The bainite wire coil spring of claim 13 wherein the wire has a tensile strength of between 200 and 270 ksi, a yield strength of between 175 and 230 ksi and a high ductility as measured by reduction in area of about 35% to about 60%.
 16. A heat treated high carbon or alloy steel bainite wire coil spring produced by the steps of: a. initially heating a continuous length of high carbon or alloy steel wire to a temperature of about 1600 degrees F. to about 1800 degrees F.; b. rapidly cooling the wire to an intermediate point temperature of between 400 to about 1000 degrees F.; c. holding the wire at the intermediate point temperature for about 2 minutes to about 40 minutes; and d. cooling the resulting lower bainite heat treated wire to ambient temperature; and e. winding the heating lower bainite wire into a coil spring.
 17. The bainite wire coil spring of claim 16 including the step of initially heating the continuous length of high carbon or alloy wire to a temperature of between 1625 degrees F. and 1700 degrees F.
 18. The bainite wire coil spring of claim 16 wherein the step of in-process cooling the wire includes cooling the continuous length of high carbon or alloy wire to an intermediate point temperature of between 550 and 950 degrees F.
 19. The bainite wire coil spring of claim 16 wherein the step of initially heating high carbon or alloy steel is for between 10 seconds and 2 minutes.
 20. The bainite wire coil spring of claim 16 wherein the step of holding the wire at the intermediate temperature is for between 10 and 20 minutes for complete lower bainitic transformation. 